Two-photon absorption coefficient in p-terphenyl crystal determined through three-photon carrier generation

Charge carrier generation in p-terphenyl crystal is studied employing pulsed excitation with a dye laser. The number of photogenerated cariers is found to be proportional to I³, where I is the intensity of the exciting light at 590 nm. The fluorescence intensity is proportional to I² under the same conditions. The results are interpreted in terms of the photoionization of two-photon excited singlet excitons. A coefficient of (2.1 ± 0.8) × 10^?50 cm⁴ s/photon for the two-photon absorption at 5 90 nm in p-terphenyl crystal is obtained.     

Spectra and structure of small ring compounds: XXIX. Microwave spectrum of γ-butyrolactone

The microwave spectrum of γ-butyrolactone has been recorded from 12.4 to 40.0 GHz. Both A-type and B-type transitions were observed. The R-branch assignments have been made for the ground state and the first two excited states of the ring-puckering and the first excited state of the ring twisting modes. It is shown that the ring skeleton is non-planar from the magnitude of the μ_c component of the dipole moment as well as from the value of I_c?(I_a+I_b). From the relative intensity measurements of the ground and the excited state, the ring twisting mode appears to be governed by a double minimum potential. The dipole moment was determined to be 4.27±0.03 D with components of μ_a = 4.04±0.03 D, μ_b = 1.42±0.03 D, μ_c = 0.33±0.02 D. From an investigation of the Raman spectrum of the gas, the ring puckering vibration was found to have a frequency of 148 cm^?1, whereas the ring twisting mode was found at 225 cm^?1.     

Emission studies of pyrene solutions

Emission spectra of pyrene in hexane have been obtained over a temperature range (from 130 to 260 K) that has not been explored before for concentrations ranging from 10^?4 mole/? to 2 × 10^?2 mole/?. A conventional and new approximation which does not depend on the experimental set-up response functions has been used for evaluating pyrene excimer association energy W_DM and other pyrene parameters. In both methods W_DM agrees quite satisfactorlly, at all concentrations used, with that reported in the literature and obtained by other techniques. However, both approximation fail to yield the right values of the rate parameters at concentrations ? 2 g/?. This set the upper limit of sample solubility to be at C = 2g/? for our range of temperature. Furthermore, the new approximation can probably be used at higher concentration (for higher range of temperature) and even may be used for other organic molecules. There was also no difference in the ratio of the excimer (I_D) to monomer (I_M) quantum yields when an intense laser beam was used as a source of excitation rather than a super-pressure Hg-lamp. This suggested that the concentrations we used might not be large enough. As a result, the laser beam would not create enough density of excited molecules to affect the ratio of I_D/I_M.     

Optical detection of nuclear spin polarization via cross relaxation in A p-dibromobenzene single crystal

Spin polarization of ⁸¹Br (I = $\text{3}\text{2}$) nuclei is achieved via cross relaxation between electronic spins of the excited triplet state of a quinoxaline guest molecule and nuclei on neighbouring molecules in a p-dibromobenzene host crystal. The cross relaxation rate is of the order of 10⁶ s^?1 and is driven by the intermolecular hyperfine interaction. Additionally, NQR transitions have been induced in the single ground state and have been optically detected by means of an optical pumping cycle involving nuclear spin polarization.     

Reactions of photogenerated fluorine atoms with dopant molecules in solid argon

The products of UV photolysis of ternary Ar?CH₄(CD₄)?F₂ mixtures (1:c:c ₀,c, c ₀=0.001–0.01) at 13–16 K were identified by ESR and FTIR spectroscopy. These products are^?CH₃ (^?CD₃) radicals of typesI andII and molecular CH₃F?HF complexes. The latter were characterized by the IR bands of the stretching C?F (1003 cm^?1) and H?F (3774 cm^?1) vibrations. The ESR spectra of radicalsI are asymmetric. The anisotropy of theg-factor (Δg～10^?3) of radicalI indicates that the structure of the radicals is nonplanar. The ESR spectrum of the typeII radical is identical to that of matrix-isolated^?CH₃ (^?CD₃) radicals with the planar structure (Δg<5·10^?5). Under the experimental conditions, the amount of complexes formed in the photolysis is equal to 0.022·c. When the photolysis is ceased, radicalI disappears after ≈10³ s and radicalII is stabilized. The limiting concentrations of the stabilized^?CH₃ and^?CD₃ radicals are equal to 2·10^?2·c and 2·10^?3·c, respectively. A mechanism of the formation of the products is suggested. It is based on the assumption that both matrix-isolated CH₄ and F₂ and their heterodimers CH₄?F₂ are present in the samples and it takes into account the long-range migration of translationally excited flourine atoms. The CH₃F?HF complexes and radicalsI are generated by the photolysis of the CH₄?F₂ heterodimers. The decay of radicalsI is caused by geminate recombination of proximate F...CH₃ pairs. RadicalsII are formed in the reaction of translationally excited fluorine atoms with isolated CH₄ (CD₄) molecules.     

Two-step picosecond UV excitation of polynucleotides and energy transfer

We studied the dependence of the efficiency of two-step photoreactions in several polynucleotides: poly-U, C_spd-phage DNA and TMV RNA, on the chain length and spatial structure of the excited biopolymer under high-intensity picosecond UV irradiation (λ = 266nm, I = 10¹¹ ?10¹⁴ W/m², τ_p = 23 ps). It has been found that the effective energy transfer distance Λ in single-stranded polynucleotides is ≈ 1 or 2 nucleotides. In double-stranded nucleic acids Λ < 170. The lifetime τ₁ of the excited electronic state S₁ and the absorption cross section σ₂ from the level S₁ have been estimated for nucleotide pU (τ > 6.6 ps, σ₂ <1.7 × 10^?16 cm²).     

Measurement of state-to-state rotational transfer rates in collision of I2*(B 0u+, υ = 15, j) with 3He, 4He,Ne, Ar,H2, D2 and I2

The selective laser excitation and induced fluorescence observation technique has been used to study rotationally inelastic collisions of I₂^*(B 0_u⁺, υ = 15,j) with I₂, ³He, ⁴He, Ne, Ar, H₂ and D₂. For each collision partner, several initial rotational levels ranging from j_i = 12 up to j_i = 146 have been excited. For purely rotational transfer within the υ = 15 level, our data are perfectly consistent with energy sudden (eventually corrected) scaling laws. Thus, any thermally averaged rate constant, k(j_i → j_f), can be expressed as a function of the basis rate constants k(l → 0). Furthermore, these k(l → 0) are found to follow simple empirical fitting laws. Consequently any k(j_i → j_f) can be predicted given a set of two or three fitting parameters. Collisions with relatively heavy particles (I₂, Ar and Ne) are well described by using the inverse power fitting law k(l → 0) = b[l(l+1)]^?γ, where b = 1.7, 1.2 and 1.2×10^?10 cm³ s^?1 and γ = 1.08, 1.02 and 1.17 for I₂^*-Ne, I₂^*-Ar and I₂^*-I₂ collisions respectively. For collisions with light particles (³He, ⁴He, H₂ and D₂), k(l → 0) shows a sharp decrease with l which can be accounted for by a hybrid power-exponential fitting law k(l → 0) = b[l(l+1)]^?γ exp[-l(l+1)/l^* (l^*+1)], where b = 0.84, 0.71, 2.77 and 2.78×10^?10 cm³ s^?1l⁺ = 20.6, 23.1, 18.8 and 31.4, and γ = 0.66, 0.66, 0.78 and 0.91 for I₂^*-³He, I₂^*-He, I₂^*-H₂ and I₂^*-D₂ collisions, respectively. We confirm that the rotational transfer dynamics in heavy molecules is mainly governed by angular momentum exchange.     

A theoretical study of the excited states of the nitroxyl radical (HNO) via the equations of motion method

The excited states of the HNO radical have been studied using the equations of motion method. These calculations confirm the presence of a low-lying ³A″ state at 5485 cm^−I, which lies between the ^IA′ ground state and ^IA″ excited state.     

Microwave spectrum,centrifugal distortion,internal rotation and excited torsional states of isobutene

The microwave spectrum of isobutene has been recorded from 10 to 35 GHz. From the analysis of the ground and first two excited torsional state splittings, the following internal rotation parameters were calculated: V₃ = 2170 cal mol.^?1, V'₁₂ = ?210 cal. mol.^?1, I_α = 3.18amu Ų and angle (methyl-top to b-axis) 58.21°. Centrifugal distortion parameters were also obtained for the ground state.     

Using 4-methyl-4-phenyl-2-pentanone as a singlet probe of benzene,cyclohexane, tributylphosphate and its multiple scavenger effects

A special probe of excited singlet states was established. The Norrish type II elimination reaction of 4-methyl-4-phenyl-2-pentanone (MPP) induced by γ-rays was reinvestigated. By measuring the G value of the sensitization radiolytic product α-methylstyrene (α-MS), we obtained the G value of excited singlet states. It was found that the G value of benzene excited singlets is 1.4±0.2, the rate constant of excitation energy transfer from benzene excited singlets to MPP is (5.2±0.5)×10⁹dm³mol^-1s^-1; the G value of excited tributylphosphate singlets is 1.4±0.3, the lifetime of its excited singlets is (1.3±0.1)×10^-7s; the G value of excited cyclohexane singlets is 1.5±0.2, the rate constant of excitation energy transfer from cyclohexane excited singlets to MPP is (4.1±0.4)×10¹⁰dm³mol^-1s^-1. It was found that MPP has multiple effects in quenching the active species formed in the radiolysis of benzene and tributylphosphate, both excitation energy transfer and reaction with subexcitation electrons have occured. A yield of subexcitation electrons of benzene 4.0, that of tributylphosphate 5.0, was obtained. The kinetics of both processes were also discussed.     

Fluorescence quenching in molecular recognition by hydrogen bonding

Steady-state fluorescence and single photon timing have been used to study the effect of the presence of hydrogen bonding on the intermolecular quenching of pyrene covalently linked to a guanine-like receptor I by an aliphatic amine (N,N-dimethylpropylamine) covalently linked to cytosine derivative II. By comparing the fluorescence quenching of I by II with that of 10methylpyrene (1-MP) by triethylamine (TEA), as a model system in which no hydrogen bonding can occur, one could possibly analyze the effect of the hydrogen bonding between receptor and substrate as a quenching as it leads to a higher local concentration of donor and acceptor. While the quenching of I by II was observed with an apparent rate constant k_q of (1.78 ± 0.10) × 10⁹ M⁻¹ s⁻¹ and (8.72 ± 0.42) × 10⁸ M⁻¹ s⁻¹ in toluene and acetonitrile, respectively, no quenching could be observed in methanol. Upon excitation of 1-MP, no quenching by II could be detected in the same concentration range as used in the quenching of I. Quenching of I and of 1-MP by TEA (⩾ 10⁻² M) in toluene leads to exciplex formation with maxima centred at 540 and 514 nm, respectively. The rate constants of exciplex formation and dissociation of I with TEA were analyzed using a global compartmental analysis. The following values were obtained for the rate constants: k₀₁ = (9.70 ± 0.01) × 10⁶ s⁻¹, k₂₁ = (1.12 ± 0.003) × 10⁹ M⁻¹ s⁻¹, k₀₂ = (5.24 ± 0.01) × 10⁷ s⁻¹ and k₁₂ = (7.74 ± 0.08) × 10⁶ s⁻¹. Quenching of I by TEA in the presence of III, a hydrogen-bonding system without an alkyl amine substituent, leads to exciplex formation centred at 538 nm. The rate constant values for the exciplex formation and dissociation of I with TEA in the presence of III were: k₀₁ = (9.32 ± 0.08) × 10⁶ s⁻¹, k₂₁ = (9.32 ± 0.003) × 10⁸ M⁻¹ s⁻¹, k₀₂ = (6.16 ± 0.03) × 10⁷ s⁻¹ and k₁₂ = (21.90 ± 0.3) × 10⁶ s⁻¹. The apparent rate constants k_q for this system was (7.26 ± 0.56) × 10⁶ M⁻¹ s⁻¹. The observed decrease in the rate of exciplex formation of I with TEA in the presence of III could suggest that the guanine-like moiety in I forms hydrogen bonds with the cytosine-like moiety and this could decrease the electron affinity of I. The rate constant of exciplex dissociation increased, indicating that the exciplex is less stable in the presence of III. Because of the single exponential decay of I in the presence and absence of II and of the agreement between steady-state and transient fluorescence measurements, the information available for quantitative analysis of the association between I and II is limited.     

Synthesis,X-ray Structures,and Photoluminescence of the Octahedral Cu4I4 Cluster with Bulky Bidentate Bis(phosphanyl)amine Ligand

The reaction of C₁₀H₇-1-N(PPh₂)₂ ( 1 ) with two equivalents of CuI in acetonitrile resulted in the formation of octahedron Cu₄I₄[ 1 ]₂ complex ( 2 ). The crystal structure of 2 showed it adopted a rare octahedral arrangement. The rectangular Cu₄ plane is μ⁴-capped by two of the iodides and is placed in axial positions above and below the Cu₄-plane form an octahedron, whereas the other two iodides are bonded to two copper atoms in a μ²-fashion. The luminescence of complex 2 arises from a triplet halide-to-ligand charge transfer (³XLCT) excited state and ³CC (Cu₄I₄ cluster-centered) excited state are not involved in the luminescence by the rigid bidentate ligand 1 in spite of the short Cu^I–Cu^I bond length. Complex 2 was identified and characterized by multinuclear NMR (¹H, ¹³C, ³¹P NMR) and IR spectroscopy. Crystal structure determinations of 1 and 2 were carried out.     

Quasiresonance charge exchange of hydrogen isotopes mesic atoms in excited states

Processes of charge exchange of hydrogen isotopes mesic atoms in excited states at low collision energies 10^?2?E?1 eV are studied. The cross sections calculated depend on energy like ～E ^?1 and are of an order of the atomic cross sections (～10^?16 cm²). It is shown that the high rates (～10¹² s^?1) of charge exchange and thermalization of mesic atoms in excited states at the liquid hydrogen density are comparable with the rates of cascade transitions in mesic atoms.     

One-way isomerization and internal rotation in anthrylethylenes in the excited triplet state

1-(2-Anthryl)-2-(bromophenyl)ethylene and 1-(2-anthryl)-2-(4-methoxyphenyl)ethylene undergo cis→trans one-way isomerization in the excited triplet state through an adiabatic process from the cis-triplet to the trans-triplet states. The trans-isomers of these compounds undergo one-way internal rotation in the excited triplet state with an activation barrier of }7 kcal mol^?1 and a frequency factor of }10¹² s^?1, while no internal rotation takes place in the excited singlet state.     

Structure and properties of 2-[(E)-2-(4-dipropylaminophenyl)-1-ethenyl]-1,3,3-trimethyl-3H-indolium chloride

The structure of styryl dye, 2-[(E)-2-(4-dipropylaminophenyl)-1-ethenyl]-1,3,3-trimethyl-3H-indolium chloride (I), was investigated using methods such as UV-VIS, fluorescence spectroscopy, and NMR (¹H, ¹³C, APT, HMQC, COSY) and also by examining its electrochemical properties. A study of the acid-base properties revealed the existence of three different forms of the dye. The mechanisms of protolysis and hydrolysis are discussed. The reagent exists in a reactive single-charged form I ⁺ over a wide range of acidity (pH 4–11). The optimum analytical wavelength of the singlecharged form is 550 nm, where the molar absorptivity is 5.51 × 10⁴ L mol^?1 cm^?1. The values of the optimum analytical wavelength and molar absorptivity of the protolysed and hydrolysed forms are: λ _max(I-H²⁺) = 380 nm, ?(I-H²⁺) = 2.01 × 10⁴ L mol^?1 cm^?1; λ _max(I-OH) = 320 nm, ?(I-OH) = 1.12 × 10⁴ L mol^?1 cm^?1. A theoretical study of the spectral and chemical properties of I was carried out by performing quantum chemical calculations.     

Reaction of excited iodine atoms with methyl iodide: Rate constant determinations

The rate constant for the reaction I(²P_1/2) + CH₃I → I₂ + CH₃ has been reevaluated taking into account both collisional deactivation of excited iodine atoms and loss of I₂ by I₂ + CH₃ → I + CH₃I. The reevaluation is based upon data obtained (R. T. Meyer), J. Chem. Phys., 46 , 4146 (1967) from the flash photolysis of CH₃I using time-resolved mass spectrometry to measure the rate of I₂ formation. Computer simulations of the complete kinetic system and a closed-form solution of a simplified set of the differential equations yielded a value of 6(± 4) × 10⁶ 1./mole-sec for the excited iodine atom reaction in the temperature region of 316 to 447 K. A slight temperature dependence was observed, but an activation energy could not be evaluated quantitatively due to the small temperature range studied. An upper limit for the collisional deactivation of I(²P_1/2) with CH₃I was also determined (2.4 × 10⁷ 1./mole-sec).     

Spin-Hamiltonian parameters and angular distortion for the trigonal Nd3+ center in congruent LiNbO3

The 52 × 52 energy matrix related to the ground multiplet ⁴I_9/2 and the first to third excited multiplets ⁴I_11/2, ⁴I_13/2 and ⁴I_15/2 for 4f³ ions in trigonal crystal field under an external magnetic field is established. By diagonalizing the energy matrix, the spin-Hamiltonian parameters (g factor g_//, g_⊥ and hyperfine structure constants ¹⁴³A_//, ¹⁴³A_⊥, ¹⁴⁵A_//, ¹⁴⁵A_⊥) of the trigonal Nd³⁺ center in congruent LiNbO₃ crystal are calculated. The calculated results are in reasonable agreement with the experimental values. From the calculations, the negative signs of hyperfine structure constants are suggested and the angular distortion of the trigonal Nd³⁺ center in LiNbO₃, which is unable to be determined by EXAFS measurement, is obtained. The results are discussed.     

Spektrochemische bestimmung von sulfat bzw. Chlorid mit roentgenfluoreszenz nach anreicherung durch mitfällung an BaCrO4 BZW. AgSCN aus verdünnten lösungen

Sulphate and chloride are quantitatively co-precipitated from dilute solutions (10^?5- 10^?4M) with BaCrO₄ and AgSCN respectively, and determined in the precipitate by X-ray fluorescence spectroscopy. Ratios of count-rates I_S/I_Cr and I_Cl/I_Sfor the appropriate K_α-lines are used for quantitative evaluation by comparison with reference standards prepared the same way. The method enables both sulphate and chloride to be determined in 50–100 ml of tap-water with a reproducibility of about 3%.     

Two-photon sequential absorption spectroscopy of iodine-127 and 129 in the 5 eV region

The two-photon sequential absordtion of iodine to electronic states in the 5 eV energy region has been re-examined using both the ¹²⁷I₂ and ¹²⁹I₂ isotopic species. The vibrational and rotational constants for the E excited state have been determined and an RKR potential curve constructed. Further information on other excited states in this region has been obtained, and the isotope effect used to locate electronic origins.     

Microwave spectrum,structure, dipole moment and low frequency vibrations of dimethyl cyanamide

The microwave spectrum of dimethyl cyanamide has been recorded from 18.0 to 40.0 GHz. Only A-type transitions were observed. The R-branch assignments have been made for the ground state and the first excited state of the skeletal bending mode. It is shown that the heavy atom skeleton is non-planar from the magnitude of the μ_c component of the dipole moment as well as from the value of I_a+I_b?I_c. The following structural parameters were obtained: r(N-CN) = 1.351, ∠CNC = 116°, and an out-of-plane angle of 36° with reasonable assumptions made for the structural parameters for the dimethylamino moiety and the nitrile bond. These parameters are consistent with those previously reported for the cyanamide molecule; they indicate a significant electron delocalization and a large contribution of the sp² hybridization on the apex nitrogen. The dipole moment components were determined to be μ_a = 4.7 D and μ_c = 0.8 D. From the low frequency vibrational data of the solid, a lower limit of 2.5 kcal/mole was obtained for the barrier to internal rotation of the methyl groups.